This invention relates to a system for the precise and adjustable metering of a plurality of liquids to be mixed and particularly to a high pressure and high flow rate metering system primarily for use in the plastics industry for reaction injection molding.
Reaction injection molding (RIM), or the use of rapid polymerizing reactants in liquid injecting molding is a process requiring an accurate metering of two or more heavy viscous fluids which, when intermixed in a predetermined ratio, are rapidly reactive to form a solid polymer. The metering of such viscous materials is generally performed by the application of a high pressure, often at a relatively high temperature, to force the fluids through small diameter orifices.
Many of the heavy viscous liquids and fiber-filled liquids used by the present day plastic industries must be metered from the displacement cylinders at relatively high pressures, in the order of 2,000 to 3,000 p.s.i. It is therefore necessary to employ displacement piston driving mechanisms that can apply the required forces to each piston stem without any danger of exceeding the stress limitations of the components in the mechanism. One prior art mechanism for accurately applying equal forces to two displacement cylinders is disclosed in U.S. Pat. No. 3,710,982 to Ferrari wherein a hydraulically operated piston linearly drives a shaft connected to one metering or displacement cylinder. The shaft carries a rack gear that is coupled through gears to a second rack on a second shaft that drives the piston in a second displacement cylinder. Thus, the gear ratio controls the linear velocity ratio between the two racks and hence the liquid mixing ratio. While this system is accurate and efficient, it requires heavy gears since the gear tooth load must be adequate to apply a full force to the driving piston in the second displacement cylinder. Furthermore, because of the requirement for such heavy duty gearing, a quick change transmission may be very large and impractical so that liquid ratio changes wculd require individual gear changes, a time-consuming process affecting production downtime.
Another prior art mechanism disclosed in U.S. Pat. No. 4,286,732 to James employs an electrical motor-driven rotatable driven shaft for driving each displacement piston. While this system may be quite accurate and adequate for the dispensing of liquids at relatively low pressures, a thick viscous liquid which requires the higher displacement cylinder pressures in the order of 2,000 to 3,000 p.s.i., may readily exceed the stress limitations of the gear teeth or shaft threads. It is, of course, possible to construct the teeth and threads of suitable materials and sizes that will ensure against such damage; however, the cost of this type of construction may render the task exceedingly costly and impractical.
The present invention is capable of applying very high forces to a plurality of displacement cylinder pistons while accurately controlling the displacement ratios and without applying excessive stresses to the gear train teeth or to any other components of the apparatus.